Apparently you can move the vending machine entrance with that LOL button you see there.
And the other entrance is a very difficult strafe jump that can only seem to be made by experianced jumpers. Anyway, look into it.

There are actually 5 way up that I am personally aware of. There is only 1 way for a single zombie to reach the roof while there are 4 spots that require cooperative boosting by fellow zombies making the roof quite legal.

Don't know if I'm allowed to even post on these threads, but Momoiya. Which railing are you talking about. The railnings on the left have the roof going over it, you can't stack there anways because a block comes down over the railing (allowing room for only one zombie) and the railings on the sides are too low (you would need a triple stack, i think this is the case, i can double check if you wish). The only places you can get up are the things on the crates.